Abstract
A green and facile fabrication strategy for synthesis of lignosulfonate-graphene porous hydrogel (LGPH) was designed via incorporation of lignosulfonate (LS) into graphene oxide (GO). This process was achieved by a simple self-assembly method at low temperature, with LS serving as surface functionalization agent. Benefiting from the abundant functional groups of LS and the large surface areas of graphene oxide, the prepared LGPH hydrogel displayed 3D interconnected pores and exhibited an excellent adsorption capacity for Cr(VI) (601.2 mg/g) ions dissolved in water. Importantly, the free-standing LGPH was easily separated from water after the adsorption process, and the adsorption capacities of Cr(VI) onto LGPH maintained 439.1 mg/g after 5 adsorption-desorption cycles. The cost-effectiveness and environmental friendliness of LGPH make it a promising material for removing heavy metals from wastewater.
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Facile Synthesis of Lignosulfonate-graphene Porous Hydrogel for Effective Removal of Cr(VI) from Aqueous Solution
Zhili Zhang,a Fengfeng Li,b Xingxiang Ji,b,* Jiachuan Chen,b,* Guihua Yang,b and Lucian A. Lucia b,c
A green and facile fabrication strategy for synthesis of lignosulfonate-graphene porous hydrogel (LGPH) was designed via incorporation of lignosulfonate (LS) into graphene oxide (GO). This process was achieved by a simple self-assembly method at low temperature, with LS serving as surface functionalization agent. Benefiting from the abundant functional groups of LS and the large surface areas of graphene oxide, the prepared LGPH hydrogel displayed 3D interconnected pores and exhibited an excellent adsorption capacity for Cr(VI) (601.2 mg/g) ions dissolved in water. Importantly, the free-standing LGPH was easily separated from water after the adsorption process, and the adsorption capacities of Cr(VI) onto LGPH maintained 439.1 mg/g after 5 adsorption-desorption cycles. The cost-effectiveness and environmental friendliness of LGPH make it a promising material for removing heavy metals from wastewater.
Keywords: Lignosulfonate; Graphene oxide; Adsorption; Heavy metals
Contact information: a: School of Light Industry and Engineering, Qilu University of Technology, Jinan 250353, PR China; b: Key Laboratory of Pulp and Paper Science and Technology of Ministry of Education, Qilu University of Technology, Jinan, 250353, PR China; c: The Laboratory of Soft Materials and Green Chemistry, Department of Forest Biomaterials, North Carolina State University, Raleigh, NC, 27695, U.S.A; *Corresponding authors: xxjt78@163.com; chenjc@spu.edu.cn
INTRODUCTION
Water pollution by Cr(VI) is a major global problem because of its detrimental and toxicological impacts on the environment and humans (Gao et al. 2018; Sun et al. 2019). Considering the toxicological impacts on animals and humans, water treatment is needed, and various techniques such as membrane filtration (Chitpong and Husson 2017), chemical coagulation (Yao et al. 2017), electrochemical oxidation, bioremediation, microwave catalysis (Wang et al. 2019), photocatalytic purification (Huang et al. 2017), and adsorption (Liang et al. 2019) have been devoted for wastewater treatment. Adsorption is superior to other techniques to remove heavy metal ions due to its lower operating cost, convenience of design, and effective elimination of the secondary contamination (Zhang and Zhang 2014; Wang et al. 2018; Yang et al. 2018a). A variety of adsorbents have been studied for Cr(VI) removal, including activated carbons, carbon nanotubes, functionalized magnetic nanoparticles, biomass, and synthetic polymers (Chang et al. 2017; Xu et al. 2017). Though the above mentioned adsorbents have been used for the removal of Cr(VI), most of these conventional adsorbents have limitations such as high cost of preparation or difficult recovery from wastewater. To overcome these limitations, an economical and eco-friendly adsorbent with excellent adsorbing performance is needed.
Recently, self-assembled graphene materials including hydrogels and aerogels have been investigated as adsorbents. Hydrogels with three-dimensional network structures have attracted intense interest in wastewater treatment, due to its unique physical and chemical properties (Wang et al. 2017). Graphene oxide (GO) shows good affinity with many soluble cationic or aromatic pollutants via electrostatic or π-conjugate interactions (Yu et al. 2013; Vu et al. 2017). Nevertheless, the absorption capacity is greatly reduced due to the reduction in the number of reactive groups during the self-assembly process (Xu et al. 2010). Thus, an effective method to enhance the absorption capacity is to increase the content of reactive groups of these materials. Biomass chemical components, such as cellulose, lignosulfonate, and chitosan, are often incorporated into adsorbents to enhance their performance (Akram et al. 2017). Lignosulfonate is the most popular compound due to its biodegradability, renewability, and good metal ion adsorption capacities (Myglovets et al. 2014). For example, Yang et al. (2014) prepared a lignosulfonate-graphene oxide-polyaniline (LS-GO-PANI) by in situ polymerization using lignosulfonate and graphene oxide. Li et al. (2016) reported a lignosulfonate-modified graphene hydrogel (LS-GH) fabricated from lignosulfonate and graphene oxide sheets via hydrothermal method. These reports suggested that the lignosulfonate incorporation improved the adsorption ability of heavy metal ions. For LS-GO-PANI, the electrostatic interactions between sulfonic anions on the LS chain and polyaniline units results in the active functional groups in LS that is consumed and generated large amounts of organic waste. Additionally, the tight composite structure greatly limits the availability of active surface area and thus limits adsorption. The hydrothermal method requires high temperature (180 °C), which can be regarded as unfavorable for commercial applications due to its high energy consumption.
To overcome the challenges mentioned above, a hydrogel functionalized with LS, a byproduct of the pulp and paper industry, was developed for the removal of Cr(VI) ions from aqueous solution. In this study, freestanding LGPH was prepared via an eco-friendly one-step method at low temperature. The abundant functional groups of LGPH were expected to be the active sites for heavy metals through electrostatic attraction. The effects of initial pH, adsorbent concentration, and contact time on the adsorption of Cr(VI) ions from aqueous solutions were also studied by batch experiments. The kinetic and equilibrium parameters were calculated to investigate the adsorption performance of LGPH. The LGPH exhibited ultrahigh adsorption ability for Cr(VI), which could have great potential application in removing heavy metals ions from industrial wastewater.
EXPERIMENTAL
Materials
Lignosulfonate (LS, C20H24Na2O10S2, MW 534.51), natural graphite powder (325 mesh, purity of 99.95%), vitamin C (VC, purity of 99.5%), K2Cr2O7, and other reagents were of analytical grade and obtained from Aladdin Industrial Corporation (Shanghai, China). All chemicals were used as received without further purification. The experimental Cr(VI) solution was prepared by dissolving K2Cr2O7 in deionized water.
Methods
Synthesis of graphene oxide
Graphene oxide (GO) was synthesized from natural graphite powder using a modified Hummers’s method (Hummers and Offeman 2002). To avoid aggregation of GO during the drying process, the GO sample was obtained by freeze-drying. The dried GO sample was dispersed in water by ultrasonication for 30 min to make an aqueous dispersion at a concentration of 2.0 mg/mL.
Synthesis of GPH and LGPH
The adsorbent LGPH was prepared as follows. First, LS (10 mg), VC (100 mg), and GO aqueous dispersion (10 mL) were added to a 25 mL glass flask. The reaction mixture was stirred at room temperature until it became a stable suspension. The mixture was then transferred to a 15 mL sealed glass bottle and autoclaved at 60 °C for 2 h. The autoclave was allowed to cool to room temperature, and the LGPH was removed. A control sample of GPH was prepared using the same procedure without LS addition.
Material characterization
The FT-IR spectra were obtained on a Vertex 70 instrument (Bruker, Rheinstetten, Germany) in the range of 500 cm-1 to 4000 cm-1 at a resolution of 0.5 cm-1. An atomic force microscope (AFM, Nanoscope III Veeco Co. Ltd., Santa Barbara, USA) was employed to characterize the GO samples. Samples for AFM analysis were prepared on freshly cleaved mica. X-ray diffraction (XRD) patterns of dried samples were measured with a Bruker D8 Advance diffractometer. The conditions of test were: the tube current and voltage at 20 mA and 30 kV, respectively, Cu target (λ = 0.15418 nm), and data were collected from the 2θ angular regions between 5° and 60°. The thermal stability of the dried samples was investigated using a TGA Q500 (TA Instruments, New Castle DE, USA). The samples were heated in an aluminum crucible to 650 °C at a heating rate of 20 °C/min. The surface morphology of dried samples was analyzed by scanning electron microscopy (SEM; Bruker) at 10 kV. The samples for SEM analysis were coated with a thin layer of gold (2 nm) by sputtering to promote conductivity before SEM observation. The instrument ASAP 2460 was employed to observe the nitrogen adsorption isotherms at -196 °C. The specific surface area was evaluated from the adsorption branch of the isotherm using Brunauer-Emmett-Teller theory (BET). X-ray photoelectron spectroscopy (XPS) was conducted on a Axis Ultra DLD spectrometer (Kratos, Manchester, UK) with monochrome Al Kα radiation (hν = 1486.6 eV).
Adsorption experiments
Approximately 100 mL of Cr(VI) solution and the composite (8 mg) were loaded in a 200 mL conical flask and placed it in an air table with 200 rpm stirring speed at 25 °C for 12 h to reach equilibrium. The desired pH value was adjusted using 0.1 M HCl or 0.1 M NaOH solution. After a desired adsorption period (0 to 600 min), the composite was removed gently with forceps from the solution, and the concentration of Cr(VI) after adsorption was measured by atomic absorption spectrophotometer (Z-2000, Hitachi Ltd., Tokyo, Japan).
The adsorbed amount of Cr(VI) on the adsorbent was calculated according to the following Eq. 1,
(1)
where Q is the adsorption capacity (mg/g); C0 and Ce are the Cr(VI) concentration before and after adsorption (mg/L), respectively; V is the initial volume of the Cr(VI) solution (L); m is the weight of the composite (g).
RESULTS AND DISCUSSION
Characterization of Samples
After heating the LS, VC, and GO aqueous dispersions at 60 °C for 2 h, a black cylinder-like LS-functionalized graphene hydrogel appeared at the bottom of the glass vessel, without any suspended GO sheets or LS in the solution (Fig. 1a). Figures 1b and 1c show, respectively, a representative AFM image of a GO nanosheet and its height profile. The cross-sectional analysis showed that the thickness of the GO sheets were in the ranges of 0.98 nm, which illustrated that the GO sheet was about one-atom-thick graphene oxide (Chang et al. 2010). The LGPH were prepared through a facile one-step hydrothermal method. The results indicated a well-defined and interconnected 3D porous network as revealed by SEM images of freeze-dried samples (Figs. 1d to 1e). The π-π stacking of graphene sheets plays an important role in the formation 3D network porous structure of GPH. For LGPH, besides the above factor, the π-π conjugation and hydrogen bonding between graphene sheets and LS contributed to the successful construction of 3D porous structure, favoring the transport and adsorption of metal ions (Zhou et al. 2016). Furthermore, the multilevel pore structure of LGPH provided large specific surface area (473.5 m2/g) that was higher than that of GPH (261.7 m2/g). These findings were confirmed by the nitrogen adsorption-desorption isotherms in Fig. 2a.
Fig. 1. (a) Photographs of monolithic materials of GPH (left) and LGPH (right); (b) AFM image of GO sheet; (c) GO sheet height profile; SEM micrograph of GPH (d) and LGPH (e)
The large surface area of LGPH also contributes to the high adsorption capacity. (Zhang et al. 2019a,b). During the hydrothermal reaction, LS penetrated into graphene sheets through non-covalent interactions, such as π-π conjugation and hydrogen bonding. The negatively charged LS molecules spread in the interlayer of graphene sheets, which prevents the restacking of graphene sheets through electrostatic repulsion. Consequently, the specific surface area of LGPH was increased compared with that of GPH, improving the adsorption capacity to metal ions (Yi and Zhang 2008).